Copper-based chip attach for chip-scale semiconductor packages

ABSTRACT

A semiconductor device comprising a semiconductor chip having an active and a passive surface, the passive surface adhesively attached to a substrate film by means of a multilayer composite; this composite comprising a metal foil having first and second surfaces and an adhesive layer attached on each of these surfaces. The multilayer composite has an average modulus larger than the modulus of the encapsulating molding compound used in the semiconductor device. By applying the composite to assembling face-up chip-scale devices, stress in solder joints is reduced and solder fatigue life enhanced.

FIELD OF THE INVENTION

The present invention is related in general to the field of electronic systems and semiconductor devices, and more specifically to structure and fabrication methods of chip-scale packages which use a copper-based chip-attach material to reduce stresses in solder joints.

DESCRIPTION OF THE RELATED ART

One of the major trends in semiconductor packaging is the effort to shrink the package outline so that the package consumes less area and less height when it is mounted onto the circuit board. Another powerful trend is the effort to achieve the outline reduction with minimum cost (both material and manufacturing cost) . One of the most successful approaches has been the development of so-called “chip-scale packages”. This expression is commonly used for packages which have an outline adding less than 20% to the chip area. A chip-scale package which has only the size of the chip itself is often referred to as “chip-size package”.

For assembling an integrated circuit (I/C) chip in a chip-scale package, there are two options: In the so-called “face-up” assembly, the chip is assembled so that the “passive” surface is attached, by some adhesive, to a substrate while the “active” surface, embedded with the (IC) and its plurality of input/output (I/O) contact pads, is facing away from the substrate. In contrast, in the “face-down” assembly, the chip is assembled so that the “active” surface faces the substrate and is attached, usually by solder balls, to this substrate.

The “face-up” assembly requires an adhesive between chip and substrate which should exhibit a number of characteristics, which are in part difficult to combine:

-   -   The chip attach material, preferably an epoxy, should show         tightly controllable “bleed-out” during chip attachment and         little release of solvents (“outgassing”) during polymerization         (hardening).     -   After polymerization, the attach material should be hardened to         a degree that it guarantees an almost immovable positioning of         the chip during the wire bonding process at elevated         temperatures.     -   The polymerized attach material should not change further during         the subsequent transfer molding process and the molding material         polymerization process at elevated temperatures and extended         times.     -   Since the finished semiconductor device represents a system         combining materials with different coefficients of thermal         expansion (CTE), the attach material has to withstand the         thermomechanical stresses, which naturally arise in this         multi-material system during significant temperature variations.     -   After the semiconductor device is attached to an outside part         such as a wiring board (usually by solder bumps), the chip         attach material has to withstand the thermomechanical stresses         between the device, the board attach material (solder bumps) and         the board itself in temperature variations. These stresses are a         natural consequence of the different CTE's of this assembled         system. In reverse, the chip attach material should help to         reduce thermomechanical stresses in solder joints in order to         increase the solder fatigue life and thus increase the         reliability of the device.     -   The attach material and the attach process should be low cost.

A number of different approaches for chip-scale device and package design, material selection and process conditions have been chosen by different semiconductor manufacturers for different market segments. A good overview of these approaches is presented in the book entitled “Chip Scale Package” by J. H. Lau and S. R. Lee (McGraw-Hill 1999). This overview shows that the latter two requirements listed above are particularly hard to fulfill jointly.

An urgent need has, therefore, arisen for a coherent, low-cost concept and method of attaching IC chips to substrates. The concept and the fabrication method should further provide increased solder joint fatigue life expectancy. The fabrication method should be simple, yet flexible enough for different semiconductor product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished without extending production cycle time, and using the installed equipment, so that no investment in new manufacturing machines is needed.

SUMMARY OF THE INVENTION

The present invention describes a semiconductor device comprising a semiconductor chip having an active and a passive surface, the passive surface adhesively attached to a substrate film by means of a multilayer composite; this composite comprising a metal foil having first and second surfaces and an adhesive layer attached on each of these surfaces. By applying this composite to assembling face-up chip-scale devices, stress in solder joints is reduced and solder fatigue life enhanced.

In a preferred embodiment, the multilayer composite consists of a copper foil (thickness range 30 to 150 μm) with a layer of epoxy resin/acrylic resin blend on each of its surfaces. The composite has an average modulus larger than the modulus of the encapsulating molding compound.

For a simple manufacturing process flow of the I/C chip attachment to the substrate film, the epoxy resin/acrylic resin layer facing the wafer support film (a wafer carrier) is ultra-violet sensitive (thickness range 20 to 50 μm). However, the epoxy resin/acrylic resin layer facing the passive chip surface is non-ultraviolet sensitive (thickness range 10 to 30 μm).

Solder joint fatigue studies based on Finite Element Modeling (FEM) showed a pronounced influence of the chip attach copper foil thickness on the board level reliability of device solder balls. As an example, for certain chip-scale devices, such as the Texas Instruments MicroStar™ Ball-Grid Array (BGA), a copper foil thickness of about 50 μm relieves the thermomechanical stress on solder balls so much that 800 temperature cycles of −40° C. to 125° C. can be passed with 1.0% failure rate. This result is equivalent to the one obtained for a hardened, epoxy-based chip attach material of about 100 μm thickness. Increasing the copper foil thickness to 75 μm pushes the board level reliability 1000 temperature cycles.

The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified and schematic cross section of the multilayer composite attachment film according to this invention.

FIG. 2 is a graph of measured modulus data of epoxy/ acrylic blend resin (in MPa) as a function of temperature (in ° C.).

FIG. 3 shows a schematic cross section of the multilayer composite attachment film positioned on a support film.

FIG. 4 is a graph of measured weight loss data of epoxy/acrylic blend resin (in %) as a function of temperature (in ° C.).

FIG. 5 is a graph of measured chip shear adhesion strength data (in kg/chip) as a function of chip size (in mm²).

FIG. 6 is a graph of measured chip shear adhesion strength data (in kg/chip) as a function of the chip attachment temperature (in ° C.).

FIG. 7 illustrates the modeled relationship of board level reliability (in temperature cycles to 1.0% failure) of a chip-scale device having a chip attach material according to the invention as a function of the copper foil thickness (in μm) used in the attach material.

FIGS. 8 to 15 are schematic and simplified cross sections of the parts and successive stages in the assembly of a chip-scale device, when attach materials and methods are used according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic cross section of the multilayer composite attachment film, generally designated 100, for use in assembling a semiconductor chip onto a substrate. The metal foil 101 has first and second surfaces, designated 101 a and 101 b, respectively. An adhesive layer 102 is attached to surface 101 a, and another adhesive layer 103 is attached on surface 101 b. For the application of the present invention, the modulus of the chip attach material should be high enough to improve fatigue life of the solder joints of the chip-scale device. Specifically, the materials of metal foil 101 and adhesive layers 102 and 103 have to be selected so that the multilayer composite has an average modulus greater than the modulus of the polymerized encapsulation material employed to complete the assembly of the chip-scale semiconductor device. For molded packages fabricated with a filler-enriched epoxy molding compound, the modulus of the encapsulation material is approximately 20 to 26 GPa.

A preferred choice for metal foil 101 is copper in the thickness range from about 30 to 150 μm. When rolled copper is used, its elastic modulus in this thickness range is approximately 200 GPa; it has a fracture strength of about 44 MPa and an elongation capability of about 25%.

The adhesive layers 102 and 103 are epoxy resin and acrylic resin blends with a modulus of about 1 GPa. Layer 102 is to be attached to the semiconductor chip and has a thickness preferably between 10 and 30 μm. For the assembly process flow of the present invention, layer 102 should be non-ultraviolet (non-UV) curable. Layer 103 is to be attached to a substrate film and has a thickness preferably between 20 and 50 μm. For the assembly process flow of the present invention, layer 103 has to be ultraviolet (UV) curable. The UV-curable epoxy resin and acrylic resin blend includes urethane resin, polyester, and ketonic resin. As usual, for semiconductor applications stringent purity requirements apply. Hot water extraction ion density values should indicate values for:

-   -   K: 0.12 ppm or less     -   Na: 0.40 ppm or less     -   NH4: 10.90 ppm or less     -   F: 1.70 ppm or less     -   Cl: 8.20 ppm or less

The UV-adhesive layer has to serve two purposes: First, it needs to provide a secure positioning of the semiconductor wafer on a UV-transparent support film for the process step of wafer dicing (singulating discreet chips) with a saw. In an initial phase, the multilayer composite 100 is placed on the support film 301 with the UV-adhesive 103 resting on support film 103, as illustrated in FIG. 3. The support film 301, about 100 μm thick, contains PVC, PET, polypropylene, and other polymer building blocks. After completion of the wafer dicing step, the adhesive 103 is exposed to UV irradiation, which is directed from the outside through the transparent transport film 301. The radiation reduces the adhesive strength of the UV-sensitive adhesive. The discreet chips can then be lifted from the support film with a vacuum pen for transport to a substrate film.

The second purpose of the UV-adhesive is to provide permanent adhesion of the singulated chip to the substrate film, after the chip has been positioned onto the substrate. In order to stabilize this positioning, the adhesive has to be hardened by polymerization (“curing”) For a typical epoxy/acrylic blend resin, FIG. 4 presents the weight loss (in %) due to outgassing during the polymerization of a typical epoxy/acrylic blend resin. Curve 401 indicates constant weight loss up to a temperature of about 300° C., and diminishing weight loss at higher temperatures. In this example, the curing was performed at 160° C. for 30 min at a temperature ramp of 5° C./min.

After this curing, a shear test may be performed to evaluate the shear adhesion strength between the chip and the substrate. In order to select unambiguous test conditions, it is practical to perform the test with a copper substrate, 300 μm thick. Examples of useful test conditions are: 100 g force applied to chip at 250° C. temperature, 30 s hold time (chip shear adhesion is measured after epoxy/acrylic blend resin was cured at 160° C. for 30 min).

Examples of acceptable results are displayed in FIGS. 5 and 6. In FIG. 5, the chip adhesion shear strength (in kg/chip) is plotted as a function of the chip size (in mm²). As curve 501 indicates, the shear strength increases far less than linearly with chip size. In FIG. 6, the chip adhesion shear strength (in kg/chip) is plotted as a function of the chip attachment temperature (in ° C.). As curve 601 indicates, the adhesion strength reaches a high, constant value only at an attachment temperature of approximately 100° C. or higher (for this investigation, a chip of 2×2 mm chip size was used; the chip attachment load was 100 g; the epoxy/acrylic blend resin was cured at 160° C. for 30 min).

The modulus of the epoxy/acrylic blend resin is a strong function of temperature. As an example for a specific blend material, the modulus-temperature relation is shown in FIG. 2 by curve 201 for a testing frequency of 1.6 Hz and an increasing temperature ramp of 3° C./min. Note that in FIG. 2 the modulus is plotted on a logarithmic scale and the unit of the modulus numbers is MPa.

The average modulus Eav (in GPa) can be expressed by the following equation: Eav=(Enon-uv·Anon-uv+Ecu·Acu+Euv·Auv)/Atot; where

-   Enon-uv is the modulus of the non-UV adhesive (in GPa); -   Euv is the modulus of the UV adhesive (in GPa); -   Ecu is the modulus of the copper foil (in GPa); -   Anon-uv is the area (=length·width) of the non-UV adhesive (in mm²) -   Auv is the area (=length·width) of the UV adhesive (in mm²); -   Acu is the area (=length·width) of the copper foil (in mm²); -   Atot is the area (additive) of the multilayer composite (in mm²)

As an example, for the modulus data listed above for the copper foil (0.03 mm thickness, 200 GPa) and the two epoxy/acrylic blend resin layers (0.01 mm and 0.02 mm thickness respectively, 1 GPa) at a 6×6 mm area size, the average modulus is calculated as 100.5 GPA. This value is significantly higher than the 20 GPa determined for the molding compound used in the example chip-size package.

For a given area size, various combinations of copper foil thickness and adhesive thickness can be used in finite element modeling to study the tensile and shear strains in solder balls at various positions in the device, when the device is attached to a board and subjected to temperature cycling from −40 to 125° C. In order to determine “board level reliability”, the number of temperature cycles is monitored, at which the solder balls reach 1.0% creep (relaxation) failures, as a function of copper foil thickness. Of particular interest. is a comparison of solder balls located under the chip and at the device package corners. As an example, a summary of these results is displayed in FIG. 7 for the following particular parameters of a preferred embodiment:

-   -   Molded chip scale ball grid array device such as TI Japan         MicroStar BGA having a solder ball pitch of 0.5 mm     -   Chip size 6×6 mm     -   Copper foil of composite chip attach 6×6 mm size, thickness at         various values from 25 to 100 μm     -   Chip attach material thickness 10 μm     -   Substrate thickness 50 μm, substrate via for solder ball 280 μm     -   Printed circuit board thickness 0.8 mm

In FIG. 7, the number of temperature cycles (−40 to 125° C.) for 1% solder ball failure, as an indicator of board level reliability, is plotted as a function of the copper foil thickness (in μm) in the chip attach material. Curve 701 represents the behavior of the solder balls under the corner of the chip; curve 702 represents the behavior of the solder balls at the corner of the device package; curve 703 represents the behavior of the device when uniform 100 μm thick epoxy resin chip attach material is used (no copper).

The curves of FIG. 7 depict that for thin copper foils, the solder balls under the corner of the chip show earlier failure than the solder balls around the corner of the package. In contrast, for thick copper, the solder balls under the corner of the package show earlier failure than the solder balls under the chip. Curves 701, 702 and 703 indicate the important result that a device with a copper foil thickness of about 50 μm in the composite chip attach accomplishes a board level reliability performance similar to the device with a 100 μm thick epoxy-based chip attach paste. Furthermore, increasing the copper foil thickness to 75 μm may push the board level reliability to the level of 1000 temperature cycles before 1% failures are reached.

FIGS. 8 to 15 illustrate significant steps of the manufacturing process flow of the multilayer composite, the wafer assembly and the chip assembly to create a semiconductor device having a chip-scale package.

-   -   FIG. 8: Providing a metal foil 801 having first and second         surfaces 801 a and 801 b, respectively;     -   attaching an adhesive layer on each of said surfaces; preferred         method is attaching a non-UV curable adhesive layer 802 on metal         surface 801 a, and attaching an UV-curable adhesive layer 803 on         metal surface 801 b; thereby creating a multilayer composite 804         having an average modulus greater than the modulus of a         polymerized device encapsulation material;     -   placing the composite film 804 with the UV-curable adhesive         layer 803 onto a transparent support film 805;     -   FIG. 9: providing a semiconductor wafer 901 having an active         surface 901 a and a passive surface 901 b;     -   attaching passive surface 901 b of semiconductor wafer 901 onto         the UV-curable adhesive layer 802 of the composite film 804;         preferred chip attachment conditions are temperature 100° C. and         higher, and force 100 g/chip;     -   FIG. 10: Shining ultraviolet light 1001 through the transparent         support film 805 on the UV-curable adhesive film 803 in order to         reduce the adhesive strength between the composite film 804 and         the support film 805; preferred UV illumination intensity is         about 120 mW/cm² and higher, UV luminous intensity about 70 to         200 mJ/cm²     -   FIG. 11: Dicing wafer 901 and attached composite film 804 into         singulated chips 1101;     -   FIG. 12: Lifting singulated chips 1101, one by one, from support         film 805;     -   FIG. 13: Providing an insulating substrate film 1301, integral         with electrically conductive routing lines, a first plurality         1302 of terminals on one surface of this substrate 1301, and a         second plurality 1303 of terminals on the opposite surface of         the substrate;     -   picking one singulated chip 1101 at a time from support film 805         and attaching the UV-cured surface 803 b of each singulated chip         (see also FIG. 12) to the substrate film 1301 (notice enlarged         lateral scale in FIG. 13 compared to FIG. 12);     -   FIG. 14: Polymerizing (“curing”) the adhesive layers 802 and         803, creating hardened layers;     -   FIG. 15: Wire bonding 1501. the active surface 901 a of each         chip to the first plurality 1302 of terminals on the substrate         1301, respectively;     -   encapsulating each chip in molding compound 1502 so that the         active chip surface 901 a, the bonding wires 1501, and portions         of the substrate film 1301 are protected;     -   attaching solder balls 1503 to the second plurality 1303 of         terminals on the substrate film 1301; and     -   singulating the substrate film 1301 to create individual devices         with outlines of chip-scale packages.

While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the semiconductor chip may be made from a material selected from a group consisting of silicon, silicon germanium, gallium arsenide, or any other semiconductor material used in integrated circuit fabrication.

As another example, the choice of the metal foil in the composite chip attach film may be any metal with a modulus so that the composite modulus is greater than the modulus of the selected molding compound. Examples include, but are not limited to, nickel, zinc and aluminum.

It is therefore intended that the appended claims encompass any such modifications or embodiments. 

1-18. (canceled) 19: A method for assembling a semiconductor device, comprising the steps of: providing a multilayer composite attachment film comprising a metal foil having first and second surfaces, a non-ultraviolet-curable adhesive layer attached on said first surface, and an ultraviolet-curable adhesive layer attached on said second surface; providing a semiconductor wafer having an active and a passive surface; placing said multilayer composite attachment film with said ultraviolet-curable adhesive layer onto a transparent support film; attaching said passive surface of said semiconductor wafer onto said non-ultraviolet-curable adhesive layer of said multilayer composite attachment film; and shining ultraviolet light through said transparent support film on said ultraviolet-curable adhesive layer in order to reduce the adhesive strength between said multilayer composite attachment film and said transparent support film. 20: The method according to claim 19 further comprising the steps of: dicing said wafer and said multilayer composite attachment film into singulated chips; providing an insulating substrate film, integral with electrically conductive routing lines, a first plurality of terminals on one surface of said insulating substrate film, and a second plurality of terminals on an opposite surface of said insulating substrate film; picking one singulated chip at a time from said transparent support film and attaching said ultraviolet-curable adhesive layer on each singulated chip to said insulating substrate film; curing said adhesive layers, creating hardened layers; wire bonding said active surface of each chip to said first plurality of terminals on said insulating substrate film with bonding wires; encapsulating each chip in molding compound so that said active surface of each chip, said bonding wires, and portions of said insulating substrate film are protected; attaching solder balls to said second plurality of terminals on said insulating substrate film; and singulating said insulating substrate film to create individual devices. 21: The method according to claim 20 wherein said step of singulating said insulating substrate film creates devices with outlines of chip-scale packages. 